Purpose
Corrections within processing software such as the Back-Projection Algorithm (for CT or NM/PET) or the Fast Fourier Transform (for MRI) are unable to overcome or remove the century-old Penumbra Effect. But this effect may be overcome by the following post-processing Computer Aided Detection (CAD) algorithm.The Penumbra-Effect (refer to Figure1 (Figure A) for the definition of Penumbra-Spread) causes blurring at the image edge-profile due to the size of the energy-source. A CAD software based on the De-convolution Technique can be used to pinpoint the true-edge positions...
Methods and Materials
(I) Phantom-Calibrations -A New York Catphan 500 was examined using Single Slice helical CT using technique factors of 120 kVp, 300mA, 4 secs., 10mm slice.Penumbra Spread, distance, area/volume, high-contrast resolution, and processing time were measured:Coin & Wire Phantom was also used to study the removal of Penumbra Effect at the edges of a Guide Wire.(II) Seven Field-Studies:Cerebral MR Angiogram of a 66 yr-male patient was used to accurately measure the size of an anterior communicating artery aneurysm by an indirect method. Data at various angles...
Results
Calibrations – Punumbra spread, Distance, Area/Volume Accuracies-Punumbra spread = 2.4 pixels (or 1.30mm for pixel size=0.54mm) across using a 1.2mm X-ray tube-spot.Distance – At high-contrast & 1% low-contrast edge, both accurate to 1/50th of a pixel in (delta)r (i.e. (delta)r in (delta)r/r);and,Area/Volume – At high-contrast & 1% low-contrast edge, accurate to 1/18th & 1/10th of a pixel respectively in (delta)r (i.e. 2(pi)r(delta)r/(pi)r2 or(delta)(area) = 2(pi)r(delta)r (Volume = Area x Slice-thickness).High-Contrast Resolution – Mean Modulation Transfer Function (MTF) (Reference 2) at the high-contrast edge were improved...
Conclusion
The century-old penumbra effect has been removed to 're-focus' the blurred image edge-profiles without any trade-off loss in signal-to-noise. Most scanner images with the penumbra effect may be clear enough for most qualitative or semi-quantitative diagnoses. But some images may require such specialized enhancement techniques to achieve diagnostic & accurate measurement goals.These case studies may have identified a number of such applications.
References
(1) "A De-Convolution Technique used for NDT in X-ray & CT". K.M.Chui et al., Oral/Full paper presentation in 17th World Conference on Non-destructive Testing (17WCNDT), Shanghai, China, October 25-28, 2008. (NDT.net - The e-Journal & Database of Nondestructive Testing - ISSN 1435-4934).(2) "The line spread function and modulation transfer function of a computed tomographic scanner". Judy P.F., Med. Phys. Vol.3, No. 4, Jul/Aug. 1976, pp.233-236.(3) "Accurate Measurements using a new algorithm in a patient with hepatic metastases from colon cancer". S.L.Chui et.al, Eurorad Case Report...
Personal Information
1) Acknowledgment – The authors wish to thank Mr. R.S.C.K. Kerr, BSc MS FRCS of Oxford John Radcliffe Hospital for his permission to use image data for the field-studies of MR & CT Angiograms in (1) & (2);2) Technical Inquiries – For technical inquiries, please contact Dr. Kui M. Chui at E-mail:
[email protected] or
[email protected] or via www.iet.org.uk ;and,3) Power-Point Slide Set - to obtain a copy of the slide-set (with animated view of all 30 analyzed slices of MR Angiogram), please contactDr. Kui M....